1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for ring focus antennas and feed systems, and more particularly to ring focus antennas and feed systems that can operate in multiple frequency bands.
2. Description of the Related Art
It is often desirable for microwave satellite communication antennas to have the ability to operate on multiple frequency bands. In those situations where a single coaxial feed for multiple bands is desired, it can be challenging to maintain existing system specifications without changing the design of the main reflector and the sub-reflector. Further, space limitations associated with existing designs can severely restrict design options.
U.S. Pat. No. 6,211,834 B1 to Durham et al. (hereinafter Durham), concerns a multi-band shaped ring focus antenna. In Durham, a pair of interchangeable, diversely shaped, close proximity-coupled sub-reflector-feed pairs are used for operation at respectively different spectral frequency bands. Swapping out the subreflector/feed pairs changes the operational band of the antenna. Advantage is gained by placement of the shaped sub-reflector in close proximity to the feed horn. This reduces the necessary diameter of the main shaped reflector relative to a conventional dual reflector antenna of the conventional Cassegrain or Gregorian variety. The foregoing arrangement of the feed horn in close proximity to the sub-reflector is referred to as a coupled configuration.
Although Durham demonstrates how a ring focus antenna may operate at different spectral bands, sub-reflector-feed pairs must be swapped each time the operational band of the antenna is to be changed. Accordingly, that system does not offer concurrent operation on spectrally offset frequency bands. U.S. Pat. No. 5,907,309 to Anderson et al. and U.S. Pat. No. 6,323,819 to Ergene each disclose dual band multimode coaxial antenna feeds that have an inner and outer coaxial waveguide sections. However, in the case of ring focus antennas, it can be desirable for the feed to have an illumination pattern that is rotationally symmetric, with substantially equal E- and H-plane beamwidths. Further, with conventional designs it can difficult to obtain the desired gain performance or illumination required to meet system specifications.
One type of horn antenna that does produce an illumination pattern that is rotationally symmetric, with substantially equal E- and H-plane beamwidths, is known as a corrugated horn antenna. A corrugated horn antenna typically includes circumferential slots, or corrugations, along the interior walls of the antenna. The depth of the corrugations is typically ¼ of a wavelength at the operating frequency, which substantially increases the surface impedance of the wall as compared to a smooth wall. The increased surface impedance results in the corrugated horn antenna having a symmetrical radiation pattern or low cross-polarization that produces nearly equal magnetic field and electric field planes. Another advantage of the corrugated horn antenna is that it typically can be operated over a larger bandwidth as compared to a horn antenna having smooth walls.
For the foregoing reasons, corrugated horns are often used as feeds for reflector antennas or as direct radiators. Still, in the case where multi-band operation of a ring focus reflector system is required, a single corrugated horn antenna has generally proved to be unsuitable. Shaping of the radiation pattern of a corrugated horn is commonly achieved by controlling the length of the horn and/or by shaping the profile of the horn. Where the length of the horn is restricted due to space limitations, shaping of the profile is a key factor for producing a desired radiation pattern.
The profile of a corrugated horn can be optimized either by using existing data concerning the effect of conventional profiles or by creating hybrid profiles that combine one or more conventional profiles. Further optimization of corrugated horn antennas can be achieved by selectively controlling the profile and/or slot depth of each corrugation. Despite the availability of such techniques, it is not always possible to optimize a single corrugated horn antenna to produce a suitable illumination pattern at widely separated frequencies of interest. Coaxial horns, such as those disclosed in U.S. Pat. No. 5,907,309 to Anderson et al. and U.S. Pat. No. 6,323,819 to Ergene can be used to create a common feed for widely separated frequencies of interest, but do not offer the benefits provided by corrugated horn antennas.